An Integrated Multi-Disciplinary Analyses to Predict the Initial Fluid Contact and Its Impact on Field Development Strategy: A Case Study from Shallow Offshore Niger Delta Field

Abstract

Estimation of initial Hydrocarbon-Water-Contact in the light of conflicting information is critical to overall field development plan and project economics. Understanding the causes of conflicting information requires an integrated investigation of potential sources of the information and a structured approach for developing and ranking plausible explanations. R11 reservoir in Yama field has conflicting information from two wells drilled 19 years apart, which indicates the possibility of compartmentalization within the reservoir or pressure depletion from a nearby field. The discovery well, which is closer to the crest, encountered a GDT and GWC is estimated to be at 9790 ftss with an assumed regional water gradient. A later appraisal well, located at the flank of the structure, clearly logged a GWC, but it is 126 ft. deeper than the estimated GWC from the discovery well. In addition, the appraisal well recorded 120 psi lower pressure than the discovery well. This conflicting information could be explained by 4 possible scenarios: 1) Errors in measurements, 2) Uncertainty in regional water gradient line 3) Reservoir compartmentalization 4) Pressure depletion from a nearby producing field. While the vintage of tool for pressure and depth measurement in the discovery well is relatively old, the difference of 126 ft. due to measurement error is unlikely. All the pressure points align quite well and estimated depth measurements errors are much smaller. On the other hand, pressure analyses with a range uncertainty in regional water gradient could narrow the difference, but not all of it alone. Possibility of reservoir compartmentalization due to faulting has quite a large impact in overall field recovery and the reservoir development. The last possible explanation for the conflicting information is the potential depletion of the pressure from a nearby field via a connecting aquifer. Analyses indicate that while it is possible to lower the reservoir pressure due to production from a nearby field, the magnitude of the pressure-drop would have been significantly smaller. Based on a series of static and dynamic modelling as well as sensitivity runs, the conflicting information can be explained by pressure depletion and the reservoir is in communication. This scenario is selected as the "Base Case" for development planning. On the other hand, the compartmentalized scenario is also considered a possibility but as a "Low Case" scenario, hence it is included in field development scenarios and the impact of it is built in the project economics.